1. Field of the Invention:
This invention relates to range finding systems, and more particularly to a range finding system of the so-called radiation beam projection type in which a radiation beam is projected toward an object, and the object distance is detected on the basis of the reflected radiation beam coming from the object.
2. Description of the Prior Art:
In the apparatus having a radiation projector by which a near infrared light is projected toward an object and a light sensor arranged in a base length from the projector to receive the reflected light, thereby the distance to the object is detected and the focus detection of a picture-taking lens, or furthermore its automatic focusing adjustment is performed, if the portion of the object which is irradiated by the projected light does not have a uniform distribution of reflection for the near infrared light (hereinafter sometimes called "contrast pattern"), a range finding error will be produced because the center of gravity of the reflected light deviates from the center of the reflected light beam.
To compensate this range finding error due to the contrast pattern, the following methods are considered. One of them as employed in the system of the type in which the reflected light is received by a pair of light sensors and their outputs are compared with each other to detect the distance, is to use an additional pair of such light sensors in combination with a subtractor for producing an output representing the difference between the differential outputs of the two pairs of sensors. Because the use of this method doubles the circuit scale, there are disadvantages in the electrical power, assembling size, compactness, the number of adjusting operations and cost. Another method as employed in the system of the type described, is to utilize the difference between the ones of the outputs of the two sensors which are produced when the projector such as a light-emitting diode, gives off no light. As the available lights range from the sun light of constant intensity to artificial lights of alternating intensities from illuminating devices supplied with current from the commercial power source, and further have a wide range of wavelengths, the dynamic range is found to be insufficient. Therefore it is difficult to put the method to practice.
Another problem of these methods is that as the size of the field of view of the sensor generally amounts to about 10 times that of the area of the radiation projection spot, the contrast of the portion which is irrelevant to the range finding is detected, thereby the contrast ratio and the range finding error are increased in magnitude.